The present invention pertains generally to devices and methods for the destruction of high level radioactive waste. More particularly, the present invention pertains to devices which use neutrons to transmute high level radioactive waste into more stable, less radiotoxic materials. The present invention is particularly, but not exclusively, useful for transmuting minor actinides, toxic fission products and plutonium into stable isotopes in a single process.
It is well known that spent nuclear fuel is highly radiotoxic and poses several challenging threats to mankind including; nuclear proliferation, radiation exposure and environmental contamination. To date, approximately 90,000 spent fuel assemblies containing about 25,000 tons of spent radioactive fuel are stored in the United States. Additionally, spent fuel assemblies are generated each year, so that it is estimated there will be about 70,000 tons of spent fuel waste by the year 2015. Since the United States currently has no permanent storage facility in operation, this high level radioactive waste is stored xe2x80x98temporarilyxe2x80x99. About 95% of this radiotoxic material is temporarily stored at the point of generation (i.e. at the power plant), awaiting a long term solution. At the power plants, the high level radioactive waste is primarily stored in water pools, with a small amount being stored in dry storage (casks). One long-term solution requires burying the waste in containers which are required to retain their integrity for at least tens of thousands of years. In addition to cost and feasibility problems, proposed burial sites have met with staunch local opposition.
Instead of burying or storing radioactive waste, another solution is to transmute high-level radioactive waste into one or more stable, less radioactive isotopes. One source of high-level radioactive waste that is of particular concern here is the spent fuel removed from a typical commercial nuclear power plant. Generally, this spent fuel contains four major constituents; uranium (about 96%), plutonium (1%), minor actinides (0.1%) and fission products (balance). The uranium and a portion of the fission products become no more radiotoxic than natural uranium ore in a relatively short time, and consequently, do not require special burial or transmutation. The remaining constituents including the plutonium, minor actinides and a portion of the fission products such as Iodine and Technetium (hereafter referred to as toxic fission products) require special burial or transmutation. To efficiently treat the spent fuel by transmutation, the spent fuel must be separated into the following four groups; plutonium, minor actinides, toxic fission products and non-radiotoxic materials.
It is known that, after the spent fuel has been separated, the radiotoxic constituents can be transmuted by reaction with neutrons into one or more stable isotopes. For example, the separated plutonium can first be transmuted by reaction with neutrons in a self-sustaining, critical, thermal neutron reaction. In such a self-sustaining critical reaction, a large percentage of the plutonium will transmute into more stable, less radiotoxic isotopes. Further, it is known that additional levels of plutonium transmutation (beyond that achieved in the self-sustaining critical reaction) can be obtained in a sub-critical thermal neutron reaction. In the sub-critical thermal neutron reaction, thermal neutrons (i.e. neutrons having energies of less than approximately 100 eV) must be supplied from a source such as a particle accelerator. By conducting irradiation experiments in thermal reactors, experimenters have demonstrated the capability to successfully destroy about 99% of the radiotoxic Pu-239 isotope.
Also, it is known that the minor actinides which are separated from the spent fuel can be transmuted to one or more stable, less radiotoxic isotopes. Specifically, this can be accomplished by the reaction of the minor actinides, which are considered non-fissile, with fast neutrons (i.e. neutrons having energies greater than approximately 100 eV). It is further known that fast neutrons can be generated by bombarding a spallation target with a beam of protons which are generated by a particle accelerator. Further, it is recognized in the pertinent art that toxic fission products separated from the spent fuel can be successfully transmuted into more stable, less radiotoxic isotopes by reaction of the toxic fission products with externally supplied thermal neutrons.
In light of the above, it is an object of the present invention to provide devices suitable for transmuting plutonium, minor actinides and toxic fission products in a single process. It is another object of the present invention to provide passively safe devices for the transmutation of separated spent radioactive fuel. It is yet another object of the present invention to provide devices that are capable of simultaneously transmuting both fissile and nonfissile radioactive materials. It is yet another object of the present invention to provide accelerator driven transmutation devices which are efficiently sized after taking advantage of plutonium""s ability to undergo a critical, selfsustaining thermal neutron fission reaction. Yet another object of the present invention is to provide transmutation devices which are easy to use, relatively simple to manufacture, and comparatively cost effective.
In accordance with the present invention, a transuranic transmuter for transmuting high-level radioactive waste includes a sealable, cylindrical housing having a window that allows a beam of protons to pass through the window and into the housing. A spallation target is positioned inside the housing and along the proton beam path. Fast neutrons are thereby generated when the beam of protons enters the housing and strikes the spallation target.
Conductive tubes containing minor actinide microspheres are positioned as a layer inside the housing and immediately adjacent to the spallation target. Specifically, these tubes are positioned inside the housing to partially surround the spallation target. The minor actinide microspheres are approximately 1.5 mm in diameter and coated with ceramic material. Also, a block of graphite formed with recesses to hold toxic fission products and plutonium is positioned behind the tubes containing the minor actinides to interpose the minor actinides between the spallation target and the graphite block. Like the minor actinides, the plutonium and toxic fission products are formed as 1.5 mm microspheres and coated with ceramic. Within the graphite block, the toxic fission products are positioned in recesses that are closer to the spallation target than the recesses containing the plutonium.
Helium is circulated through the housing and between the conductive tubes to regulate the temperature inside the housing. Also, the graphite block is formed with cooling channels to further allow the helium to circulate within the graphite block.
In operation, the transmutation of the radiotoxic material can be efficiently conducted in a two-step process. In the first step, a critical, self-sustaining, thermal neutron fission reaction can be initiated in the plutonium with the proton source de-energized. In the second step, further transmutation of the radiotoxic materials may be achieved with the proton source energized.
During the first step, the critical, self-sustaining, thermal neutron fission reaction initiated in the plutonium will produce fast neutrons. These fast neutrons will radiate from the plutonium towards the toxic fission products and the minor actinides. Since the plutonium is held in a moderating graphite block, the fast neutrons will pass through the moderator before reaching either the minor actinides or the toxic fission products. Nevertheless, some of these neutrons will reach the minor actinides with energies in the fast spectrum, where they will be effective in fissioning a portion of the minor actinides, and creating new fast neutrons to fission additional amounts of minor actinides. Similarly, some neutrons will reach the toxic fission products with energies in the thermal spectrum, where they will be effective in transmuting a portion of the toxic fission products. Consequently, in the first step of operation, a portion of the plutonium, the minor actinides and the toxic fission products will be transmuted into one or more stable, less radiotoxic isotopes.
In the second step of operation, a beam of protons is directed from the proton source into the housing and onto the spallation target. As the protons impact the spaliation target, fast neutrons are generated by the target which travel towards the radiotoxic materials. A portion of the fast neutrons generated at the spallation target react with the minor actinides, causing the minor actinides to transmute by either fission or neutron capture reactions into one or more stable, less radiotoxic isotopes.
The residual fast neutrons from the spallation target will enter the graphite block travelling towards the toxic fission products and plutonium. Additionally, the fast neutron fissioning of the minor actinides will generate neutrons, a portion of which will enter the graphite block travelling toward the toxic fission products and the plutonium.
While passing through the moderating graphite block the neutrons will react with the graphite and lose energy. Consequently, within the graphite block, the energies of the neutrons are, on average, highest near the spallation target. After passing through a portion of the graphite block, the moderated neutrons will react with the toxic fission products and the plutonium causing the toxic fission products and the plutonium to transmute into one or more stable, less radiotoxic isotopes. Within the graphite block, the toxic fission products are positioned closer to the spallation target than the plutonium to take advantage of the fact that the toxic fission products transmute more efficiently when reacted with the higher energy, higher flux neutrons.
The heat generated in the various transmutation processes described above is regulated and controlled by the device of the present invention in several ways. Primary temperature regulation is achieved by circulating helium through the inside of the housing. Specifically, helium is circulated inside the housing between the conductive tubes and through cooling channels formed in the graphite block. Further, the device of the present invention is designed and configured to be passively safe. Consequently, a melt-down can be avoided in the event of a helium coolant failure. For example, the minor actinides are placed in thermally conductive tubes for the purposes of conducting heat away from the minor actinides. Also, the ratio of minor actinides to plutonium charged into the transuranic transmuter can be controlled. Lastly, the reaction rates can be controlled by varying the power of the proton beam.